Method for force correction

ABSTRACT

A method for directional force correction of pneumatic tires that simultaneously corrects radial and tangential force exerted by the tire. The method allows for the correction of tangential force regardless of the rotational direction of the tire. This invention accomplishes this end through an improved force correction technique, wherein at least two rotary grinders are employed. In the inventive method, the rotational direction of one grinder is reversed in relation to the rotational direction of the other grinder(s). Therefore, at least one grinder will engage the tire in an “up-grinding” manner, and at least one grinder will engage the tire in a “down-grinding” manner. The result is that across the width of the tire&#39;s tread surface, some tread blocks will have a “heel-to-toe” appearance, while others will appear “toe-to-heel.” The effect is that, no matter what direction the tire is rotating while in use on an automobile, tangential force variation has been reduced.

BACKGROUND OF THE INVENTION

[0001] Each year, vehicle vibration is among the most common sources ofnew vehicle dissatisfaction. Any rotating component of a vehicle is apotential excitation source for vibration, from brake rotors, theengine, driveline, wheels, tires, or even the highway surface itself.Generally, this invention relates to the use of a low speed tireuniformity machine to correct for the variation of the directionalforces exerted by tires rotating against a wheel with constantdeflection. This invention specifically relates to an improved methodfor optimizing tire uniformity, wherein a process for correcting radialforce variation simultaneously reduces the directionally dependenttangential force variation caused by the uniformly sloped tread-blocksor “heel-to-toe” effect of conventional force correction techniques,thereby reducing the tangential force variation and improving the ridecomfort of the tire.

[0002]FIG. 8 displays the tire uniformity coordinate system typicallyused in the tire industry for low and highway speed uniformity machines.The system illustrates the forces and moments generated by the tire asit rotates through the tire footprint or tread surface. Force variationproduced by a tire rotating at constant deflection is broken down intothree orthogonal forces referred to in industry literature as radial(Fz), lateral (Fy), and tangential (Fx). Radial force is the verticalforce between the tire and the road surface, most typically the roadacross which the tire travels. Radial force is applied on an axis thatis perpendicular to the road, and is in effect the “up and down” forceacting upon a wheel's axle. Tangential or fore/aft force is thehorizontal force between the tire and the road. The tangential axis isparallel to the road, in the direction of travel. It is on this axisthat the tangential or driving force is applied. Tangential force iseffectively the front-to-back force that acts upon the axle. Lateralforce is the side-to-side force along the rotational axis between thetire and the road. The lateral axis is where side-to-side forces areapplied by the tire, and this axis is parallel to the road surface,perpendicular to the direction of travel. That these three forces willbe generated by a tire rotating against a load is a law of physics—theywill always be present.

[0003] A goal of tire manufacturing is to eliminate or reduce anyadverse effects that these forces may have on the ride of a tire, suchas wheel hop or vibration. The mere existence of such forces does notcreate ride disturbance per se. For example, when the forces remainconstant throughout the entire revolution of a tire, the tire's ridewill be undisturbed. It is only when the forces vary throughout thecourse of a tire's revolution that ride disturbance is observed.

[0004] To illustrate this point, consider the force of gravity.Practically speaking, the earth's gravitational force is perpetual andconstant in its magnitude. So while it is always acting upon everyobject on or near the earth's surface, it acts upon those objects in thesame way at all times. In that sense, the force of gravity does notdisturb the regular course of activity on earth. However, if the forceof gravity were to change throughout the course of a day, activity onearth would be greatly disturbed. If the earth's gravity were to change,the weight of everything on earth would change as well. It is neitherdesirable nor possible to eliminate the force of gravity, but it isdesirable that it remain constant. And so it is with the forcesgenerated by tires. It is not the forces themselves that noticeablyaffect the ride of a tire, but the variation in force that isresponsible for ride disturbance.

[0005] Force variation is generated by the rotation of a tire that isnot uniform. Two primary tire deformities result in radial forcevariation: being “out of round” or slightly misshapen, and a variationin the tire's carcass stiffness. Carcass stiffness is the measure of atire's resistance to flexing while revolving against a load. Resistanceto flexing is simply another way of describing how much force the tirecarcass is exerting against the road. Both of these deformities generatea vertical force component that disturbs the equilibrium of the wheel'saxle and causes it to undergo an up and down movement during eachrevolution of the tire. This occurs on a tire that is out of roundbecause some portions of the tread surface are simply farther away fromthe axle than others, therefore making the vertical distance between theaxle and the road dependent upon which portion of the tread surface ismaking contact with the road. Similarly, a tire with a variable carcassstiffness will cause the axle to move up and down during the course of arotation because some portion of the carcass will push less hard againstthe road than the remaining portion. If the variation in stiffness isgreat enough, the axle will be pushed farthest from the road when thestiffens portion of the carcass rotates across the road, and it willfall closer to the road when the most flexible portion of the carcasscomes in contact with the road.

[0006] Generally, many of the deformities that cause radial forcevariation also cause tangential force variation. Tangential forcevariation is generated when the angular velocity of the tire changesthroughout the course of its revolution. A change in angular velocitymeans that for a tire driven at constant rotational speed, some pointson the tread surface of the tire are traveling at a faster linear speedthan are others. This is easy to conceptualize for a tire that is out ofround. In order to be out of round, some points on the tread surfacemust be farther away from the axle than are the others. That means thatduring the course of one rotation, the point farthest from the axle willtravel a greater linear distance to complete its rotation than all ofthe other points, yet it will have done so in the same amount of time.Because speed equals distance traveled divided by time, the linear speedof the tire at the point farthest from the axle must be traveling fasterthan all of the other points. When that point makes contact with theroad, the tire in effect pulls or accelerates the axle forward. However,as soon as that point passes the road, a slower point comes into contactwith the road and acts to decelerate the axle or push it back. Thispushing and pulling motion will occur once every rotation, resulting inride disturbance. Variation in carcass stiffness will also cause thispushing and pulling effect because the least rigid portion of the tirewill travel more slowly across the road than the rest of the tire.

[0007] Because many tire deformities are generally responsible for bothradial and tangential force variation, it is logical that detecting andcorrecting the deformity would correct for both types of variation.Grinding is an effective technique for correcting tires that are eitherout of round or have variable carcass stiffness. A tire that is out ofround can be ground so that it is uniform. Grinding can also makecarcass stiffness more uniform. If carcass stiffness is thought of asthe spring force of the tire, then the goal is to make the force aconstant. A spring's force or potential is the product of its length andits spring coefficient, which is a constant unique to that spring. Ifeach point on a tire's tread surface is thought of as a spring from thatpoint to the axle, then the spring force would be the product of thelength of that spring and its coefficient. If the spring lengths are notuniform, the tire is obviously out of round, and grinding can remedythis. If the tire is uniformly circular, but of variable carcassstiffness, then the variance is in the spring coefficients. Because aspring's coefficient is constant, and the goal is to make the spring'sforce constant, the only factor than can be corrected for is springlength. Grinding minute amounts of rubber at those points of greatestspring force will reduce the spring length and serve to make the tiremore uniform without taking it so far out of round as to create ridedisturbance.

[0008] Although correcting for radial force variation should alsotheoretically correct for tangential force variation, separate detectionand correction techniques for each are known in the art. It is desirablefrom a production standpoint to accomplish both corrections by employingonly one technique, saving the manufacturer both time and money. Due tothe realities of tire manufacture, it is optimal that this dual forcecorrection be accomplished through the detection and correction ofradial force variation. Radial force variations generally existindependent of the speed of tire rotation, and force correction istypically done at very slow speeds, such as 60 rpm. Low speed uniformitymachines are used for the radial force detection and correction process,and are relatively inexpensive and in widespread use in the industry.Conversely, tangential force variation is speed-dependent, and generallycannot be detected at 60 rpm. The machine used to test and correcttangential force variation must be capable of rotating a tire at speedsof at least 300 rpm, and preferably 800 rpm or more. These highway speeduniformity machines are extremely expensive and are not commonly used inthe commercial manufacture of tires. It is therefore desirable to beable to eliminate tangential force variation simultaneously withcorrection of radial force through employment of the low speeduniformity machine.

[0009] A known tire uniformity machine utilized to correct for radialforce variation and its operation is described as follows: A motordrives a wheel to which a tire will be mounted. The tire is rotatedagainst a free rotating load drum, which is connected to forcetransducers, which feed information into a computer. The computerdirects grinding wheels to engage the tire. The grinding wheels aredriven by motors which are attached to amp meters, which measure theload on the motors. Typically, the radial force correction processoccurs as follows: a tire is mounted on a precision chuck, inflated to atest pressure, and rotated under a predetermined load against a loadingdrum. Radial force and radial force variation are then measured on theloading drum by force transducers located in the radial direction on theaxis of the loading drum. A pair of rotary grinders positioned adjacentthe shoulders of the tire tread are next moved into grinding engagementwith the tread shoulder ribs. The grinders are moved into the tire toremove rubber at high force locations. Minute quantities of rubber areground from the shoulders of the tire. The grinding results in a moreuniform tire with reduced radial force variation.

[0010] In this conventional process, the grinders are typically drivenin the same rotational direction relative to each other. The grindersmay engage the tire in either a “down-grinding” or “up-grinding” manner,depending on the rotational direction of the grinders relative to thatof the tire. “Down-grinding” is depicted in FIG. 2, and it occurs wherethe rotational direction of the grinding wheels is opposite to that ofthe tire, but at the interface of the tire and grinder surfaces, thesurfaces are moving in the same linear direction. For each tread-blockground by the down-grinding process, the depth of grinding engagementwill increase from the point of first contact. FIG. 3 illustrates theprocess of “up-grinding,” which occurs where the rotational direction ofthe grinders is the same as that of the tire, but at their interface,the surfaces of the tire and the grinders are moving in opposite lineardirections. The depth of grinding engagement by this method is deepestat the point of first contact between the grinder and the tread-block.Both methods of grinding engagement create uniformly slopingtread-blocks across the shoulders of the tire, the orientation of theslope being the primary distinction between the methods. The slopedappearance of the tread-blocks is described as “heel-to-toe” or“toe-to-heel,” depending upon the orientation of the slope. Grinderrotation relative to that of the tire dictates the slope.

[0011]FIG. 1 illustrates a typical profile of tread-blocks of anungrounded tire. When viewed from this perspective, the tread surface isflat, or has no slope. In the context of this specification and theseclaims, slope is defined as it would be in a Cartesian coordinatesystem. Slope equals the change in the vertical component of a line orplane over the change in the horizontal component of the same line orplane. Referring to the tread-blocks of FIG. 1, the vertical componentof the blocks never changes, and therefore the slope is zero. FIG. 2illustrates a tire subject to radial force variation correction whereinthe tire 2 is rotated in the clockwise direction during the correctionprocess. The grinder 4 shown is rotating in the counter-clockwisedirection, thereby engaging the tire in a “down-cutting” manner. Asshown, the resultant tread-blocks appear “heel-to-toe,” or have apositive slope 6. FIG. 3 illustrates the effect upon the tread-blocks 6when the rotational direction of the grinding wheel in FIG. 2 isreversed. Here, the grinding wheel is engaging the tire in an“up-cutting” manner, and the resultant tread-blocks appear“toe-to-heel.” Because the vertical component of the tread-block isdecreasing as the horizontal component increases, the slope is negative.

[0012] The radial force correction method described above effectivelyserves its purpose of significantly reducing or eliminating radial forcevariation. However, this technique does not always satisfy itstheoretical potential of reducing or eliminating tangential forcevariation. In fact, in most cases this method has been shown to bedirectly responsible for an increase in tangential force variation. Itwas discovered that when tires force corrected by the known radial forcemethod described above were tested for tangential force variation on ahighway speed uniformity machine, the tires generated less tangentialforce variation than ungrounded tires when rotating in one direction,but more tangential force variation when rotated in the other direction.Practically speaking, because tires on opposite sides of an automobilerotate in opposite directions, half of the tires corrected by the aboveprocess would have reduced tangential force variation, while those onthe other side would exert increased tangential force variation. It wasfurther discovered that the uniform tread-block profiles as depicted inFIGS. 2 and 3 were directly responsible for this result.

[0013] Most tires are designed to be interchangeable, and thereforeuniform to each other. The surest way of achieving interchangeabilityand uniformity is to manufacture and force correct each tire in anidentical fashion. However, with four identically manufactured andradial force corrected tires it can be observed that the tires on oneside of the automobile will exert significant tangential force variationwhile those on the other side will not. Although the tires are allidentical as they emerge from the manufacturing and correction process,one thing changes when the tires are mounted on an automobile—when tireswith identical heel-to-toe profiles are mounted on opposing sides of anautomobile, their heel-to-toe profiles reverse in relation to eachother. Therefore, when the tires on one side rotate heel-to-toe, theothers will rotate toe-to-heel. To remedy this situation, a manufacturerwould have to correct half of its tires by up-grinding, and the otherhalf by down-grinding, assuming the direction of tire rotation was keptconstant. This is not desirable because it requires the time and expenseof setting up a production line accordingly, and requires that the tiresbe sold to be mounted only on a specific side of an automobile.

[0014] The known methods for force correction result in the uniformlysloped tread-blocks as shown in FIGS. 2 and 3. The sloped tread-blocksserve to either significantly reduce or increase the tangential forcevariation upon the tire, depending on the rotational direction of thetire. A tire rotating in the “heel-to-toe” direction has been shown toexert low tangential force variation or at least reduced variationcompared to that of an ungrounded tire, because it travels more smoothlyacross the road surface than the ungrounded portion of the tire. Bycontrast, a tire rotating “toe-to-heel” has been shown to exertsignificantly higher levels of tangential force variation, because itdoes not travel as smoothly as the ungrounded portion of the tire. Thisis significant because force pulses are created as each tread block,ground or ungrounded, passes over the road surface. These pulses canserve as an excitation source of the fore/aft torsional resonance of thevehicle. When this resonance is excited, the tangential or fore/aftforce variation is observed. When the grind is in the same direction onthe inside and outside shoulder ribs, the pulses add constructively,providing a potential excitation source for nearly all of the fore/afttorsional resonance frequencies, with nearly equal magnitudes.

[0015] On any passenger automobile, the tires on the left side (thedriver side in the United States) of a forward moving vehicle rotatecounter-clockwise (when viewed from the left side), and the tires on theright side rotate clockwise (when viewed from the right side). Whentires are force corrected in the same manner, they have identicaltread-slope orientations. However, when mounted on an automobile, thetread-slope orientation on one side of the vehicle will be reversed inrespect to that of the tires on the other side. Therefore, the tires oneside rotate “heel-to-toe” and experience little or no tangential forcevariation, while the tires on the other side will rotate “toe-to-heel”and be subject to unacceptably high levels of tangential forcevariation. For example, a tire corrected by the process depicted in FIG.2 will rotate heel-to-toe when mounted on the left side of an automobilemoving forward, and will rotate toe-to-heel when mounted on the rightside. The tires on the right side will generate significant tangentialforce variation, whereas those on the left side will not. By contrast,tires corrected by the process depicted in FIG. 3 will do exactly theopposite. They will rotate heel-to-toe on the right side and toe-to-heelon the left. The tires on the left side will now generate the tangentialforce variation.

SUMMARY OF THE INVENTION

[0016] The object of the present invention is to provide for correctionof radial force variation while simultaneously reducing tangential orfore/aft force variation, regardless of the rotational direction of thetire. This invention accomplishes this purpose through an improved forcecorrection technique, wherein at least two rotary grinders are employed.In the inventive method, the rotational direction of one grinder isreversed in relation to the rotational direction of the othergrinder(s). Therefore, at least one grinder will engage the tire in an“up-grinding” manner, and at least one grinder will engage the tire in a“down-grinding” manner. Furthermore, the tire may be engaged withgreater pressure from the “down-grinding” grinder(s). The result is thatacross the width of the tire's tread surface, some tread blocks willhave a “heel-to-toe” appearance, while others will appear “toe-to-heel.”The effect is that, no matter what direction the tire is rotating whilein use on an automobile, tangential force variation has been reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a profile view of a series of ungrounded tread-blocks,as seen from the outer wall of a tire;

[0018]FIG. 2 is a diagrammatic view of a tire being engaged by grindingwheels in a “down-grinding” manner, and the resultant “heel-to-toe”profile of the ground tread-blocks, as seen from the outer wall of thetire;

[0019]FIG. 3 is a diagrammatic view of a tire being engaged by grindingwheels in an “up-grinding” manner, and the resultant “toe-to-heel”profile of the ground tread-blocks, as seen from the outer wall of thetire;

[0020]FIG. 4 is a schematic diagram of a tire being engaged by grindingwheels in the practice of the invention;

[0021]FIG. 5 is a schematic diagram of a tire being engaged by grindingwheels in the practice of the invention;

[0022]FIG. 6 is a profile view of the width of a tread surface of a tireshowing tread-blocks displaying both “heel-to-toe” and “toe-to-heel”slopes, as seen from the outer wall of the tire;

[0023]FIG. 7 is a tire block element description of the “toe-to-heel”effect on the shoulder ribs due to the two grinding wheels rotating inthe same direction and the “heel-to-toe” effect on the center ribs dueto the additional center rib grinder rotating in the opposite directionto the shoulder grinders in the practice of the invention.

[0024]FIG. 8 is a depiction of the tire uniformity coordinate systemtypically used with low speed and highway speed uniformity machines.

DETAILED DESCRIPTION OF THE INVENTION

[0025]FIG. 4 illustrates one embodiment of the invention. Shown is atire uniformity machine which is comprised of a motor 14 which drives atire 12 against a load wheel 18. Connected to the load wheel is a forcetransducer 46 which is in turn connected to a computer 48. The computercommands grinder motors 20 and 28 to driving grinding wheels 16 and 24.Also attached to the grinder motors are amp meters 22 and 30. In thepractice of the invention as shown in FIG. 4, the tire 12 as shown isrotated in in the clockwise direction, and grinding wheels 16 and 24 arerotated clockwise and counter-clockwise, respectively. Grinding wheel 16is rotating in the same direction as the tire, and the other grindingwheel 24, is rotating in the opposite direction. The grinding wheels aredriven by grinder motors 20, 28, and the load on the grinder motors ismeasured by amp meters 22, 30. The load on the grinder motors isindicative of the relative pressures with which the grinding wheelsengage the tire. Pressure is directly proportional to the load on thegrinder motors. Therefore, the greater the pressure applied to the tire,the greater the load on the grinder motor. As the pressure on the tireand the load on the motor increase, so does the depth of the grind.

[0026] In FIG. 4, grinding wheel 16 is engaging the tire in anup-grinding manner. Grinding wheel 24 is engaging the tire in adown-grinding manner. In the embodiment of the invention shown in FIG.4, the load on the down-grinding wheel's motor 28 should be greater thanthat on the motor of the up-grinding wheel 20. In other words, thepressure applied to the tire by the down-grinding wheel should begreater than the pressure applied by the up-grinding wheel. Ideally, theload on the up-grinding motor is in the range of 60 to 80% of thedown-grinding motor.

[0027]FIG. 6 illustrates the tread profile of a tire after having beenforce corrected by the process depicted in FIG. 4. Tread-blocks 34, 36,38 represent the shoulder of the tire corrected by the up-grindingwheel. From this view of tread-blocks 34, 36, 38, they have been groundso that their slope is negative, or “toe-to-heel” from left to right.Tread-blocks 40, 42, 44 represent the shoulder of the tire ground by thedown-grinding wheel. From this view of tread-blocks 40, 42, 44, theyhave been ground so that their slope is positive, or “heel-to-toe” fromleft to right.

[0028] Every tire corrected by this process will always have oneshoulder rotating heel-to-toe and one shoulder rotating toe-to-heel,regardless of the tire's rotational direction. Therefore, when grindingto correct for radial force variation by this method, no significanttangential force variation will be created in the process. Each tirewill be uniform in its own rotation, and will also be uniform inrelation to the other tires with which it will be utilized on anautomobile. The result is greater ride comfort.

[0029]FIG. 5 illustrates another embodiment of the applicant'sinvention. Grinding wheels 52 and 60 are positioned on the shoulders ofthe tire 12 and are driven in the same rotational direction relative toeach other. Grinding wheel 66 is positioned in the center of the treadsurface, and is driven in the opposite rotational direction of grindingwheels 52 and 60. In this configuration as shown, grinding wheel 66 isthe up-grinding wheel, and more pressure should be applied by it to thetire than the pressure applied by grinding wheels 52 and 60, contrary tothe invention as depicted in FIG. 4. In fact, in practicing theinvention where the two shoulder grinding wheels are rotatingidentically and the center grinding wheel is rotating in the oppositedirection, the pressure applied by the center grinder should always begreater. Ideally in the configuration of FIG. 5, the load on thedown-grinding motors is 30 to 70% of the up-grinding motor. It has alsobeen realized that a +/−40% variation in the relative differentialbetween the amperages produces similar results. In any embodiment of theinvention, the pressure differential on the grinding wheels should begoverned by the goal of grinding the tread blocks so that the pulsesgenerated by the heel-to-toe and toe-to-heel portions of the treadsurface are equal and opposite, resulting in destructive interferencewhich reduces tangential force variation.

[0030] A tire ground by the above process will have shoulders withidentical heel-to-toe orientation, while the center tread-blocks willhave the opposite heel-to-toe orientation, as shown in FIG. 7. Treadblocks 34, 36, 38, 40, 42, and 44 were all corrected by down-grinding,and their slope is positive. Tread blocks 46, 48, 50, 52, 54 and 56 werecorrected by up-grinding, and their slope is negative. The result is atire which will have heel-to-toe and toe-to-heel rotational componentsregardless of the tire's rotational direction. Such tires will havereduced tangential force variation and will provide greater ridecomfort.

[0031] While certain representative embodiments and details have beenshown for the purpose of illustrating the invention, it will be apparentto those skilled in the art that various changes and modifications maybe made therein without departing from the spirit or scope of theinvention.

What is claimed is:
 1. An improved method for reducing tangential forcevariation exerted by a pneumatic tire-rotating against a load, theimprovement comprising: (a) mounting said tire on a on a known tireuniformity machine having at least two grinding wheels; and (b)employing said uniformity machine to grind the tire whereby at least oneof the grinding wheels is engaging the tire in an up-grinding manner andat least one of the grinding wheels is engaging the tire in adown-grinding manner.
 2. The improved invention of claim 1, wherein: (a)there are two grinding wheels; and (b) said grinding wheels arepositioned to grind the shoulders of the tread surface of the tire, oropposing intermediate ribs.
 3. The improved invention of claim 2,wherein unequal grinding pressure is applied to the tire, so that thepressure applied by the grinding wheel engaging the tire in adown-grinding manner is greater than the pressure applied by thegrinding wheel engaging the tire in an up-grinding manner.
 4. Theimproved invention of claim 2, wherein the pressure applied by thegrinding wheel engaging the tire in a up-grinding manner corresponds toa load on the grinding wheel motor of about 60 to 80% of the loadapplied to the motor of the down-grinding wheel.
 5. The improvedinvention of claim 1, wherein: (a) there are three grinding wheels; and(b) two of said grinding wheels are positioned to grind the shoulders ofthe tread surface of the tire, both of said grinding wheels being drivenin the same rotational direction relative to each other; and (c) thethird of said grinding wheels is positioned to grind the center of thetread surface of the tire, said grinding wheel being driven in theopposite rotational direction relative to the grinding wheels positionedon the shoulder of the tire.
 6. The improved invention of claim 5,wherein unequal grinding pressure is applied to the tire, so that thepressure applied by the center grinding wheel is greater than thepressure applied by the grinding wheels engaging the tire on theshoulders.
 7. The improved invention of claim 5, wherein the pressureapplied by the shoulder grinding wheels corresponds to a load on thegrinding wheel motors of about 30 to 70% of the load applied to themotor of the center grinding wheels.
 8. The improved invention of claim1, wherein unequal grinding pressure is applied to the tire, so that thepressure applied by the grinding wheel(s) engaging the tire in adown-grinding manner is greater than the pressure applied by thegrinding wheel(s) engaging the tire in an up-grinding manner.
 9. Theimproved invention of claim 1, wherein the pressure applied by thegrinding wheel(s) engaging the tire in a up-grinding manner correspondsto a load on the grinding wheel motor of about 30 to 80% of the load onthe grinding wheel motor of the down-grinding wheel(s).
 10. An improvedmethod for simultaneously correcting radial and tangential forcevariation exerted by a pneumatic tire rotating against a load,regardless of the tire's rotational direction, including the reductionof the tangential force variation component that is resultant from theuniformly sloped tread-blocks of conventional radial force correction,the improvement comprising: (a) mounting said tire on a tire uniformitymachine having two grinding wheels, wherein one grinding wheel ispositioned on each shoulder of the tire; (b) employing said uniformitymachine to grind the tire whereby one of the grinding wheels is engagingthe tire in an up-grinding manner and the other grinding wheel isengaging the tire in a down-grinding manner; and (c) applying unequalgrinding pressure to the tire, wherein the pressure applied by thegrinding wheel engaging the tire in a down-grinding manner is greaterthat the pressure applied by the grinding wheel engaging the tire in anup-grinding manner.
 11. The improved invention of claim 10, wherein thepressure applied by the grinding wheel engaging the tire in aup-grinding manner corresponds to a load on the grinding wheel motor ofabout 60 to 80% of the load on the grinding wheel motor of thedown-grinding wheel.
 12. An improved method for simultaneouslycorrecting radial and tangential force variation exerted by a pneumatictire rotating against a load, regardless of the tire's rotationaldirection, including the reduction of the tangential force variationcomponent that is resultant from the uniformly sloped tread-blocks ofconventional radial force correction, the improvement comprising: (a)mounting said tire on a tire uniformity machine having three grindingwheels, wherein a grinding wheel is positioned on each shoulder of thetire, and the remaining grinding wheel is positioned in the center ofthe tread surface; (b) employing said uniformity machine to grind thetire whereby at least one of the grinding wheels is engaging the tire inan up-grinding manner and at least one of the grinding wheels isengaging the tire in a down-grinding manner; (c) driving the grindingwheels positioned on the shoulders of the tire in the same rotationaldirection relative to each other, and driving the grinding wheelpositioned in the center of the tread surface in the opposite rotationaldirection relative to that of the other grinding wheels; and (d)applying unequal grinding pressure to the tire, wherein the pressureapplied by the center grinding wheels is greater that the pressureapplied by the shoulder grinding wheels.
 13. The improved invention ofclaim 12, wherein the pressure applied by the shoulder grinding wheelscorresponds to a load on the grinding wheel motor(s) of about 30 to 70%of the load on the motor of the center grinding wheel.
 14. An improvedmethod for reducing tangential force variation exerted by a pneumatictire rotating against a load, the improvement comprising: (a) mountingsaid tire on a on a known tire uniformity machine having two grindingwheels; (b) grinding the shoulder ribs of said tire whereby the grindingwheels are rotating in the same direction relative to each other,creating a similarly sloped heel-to-toe profile across the tread surfaceof the tire; (c) determining the rotational direction of the tire whichcorresponds to the tread blocks rotating heel-to-toe; (d) determining onwhich side of a vehicle said tire should be mounted so that it rotatesheel-to-toe when the vehicle moves in the forward direction; and (e)identifying the tire so that it is to be mounted the specific side ofthe vehicle, as determined above.
 15. A pneumatic tire wherein at leastsome of the tread blocks on the shoulders of said tire have opposingsloped heel-to-toe profiles.
 16. A pneumatic tire wherein at least someof the tread blocks on the shoulders of said tire have similarly slopedheel-to-toe profiles, and the tread-blocks in the center of the treadsurface have a heel-to-toe profile which is of opposite slope relativeto that of the shoulder tread blocks.